There are known centrifugal classifiers of various types. One (cf. USSR Inventor's Certificate No. 740305, Cl. B07B, 7/083, 1980) has a housing contained wherein is a rotor with discs which whirl a gas/powder mixture as this passes through the so-called separation zone (a space where the material is separated into fractions) formed by the edges of the discs. The larger particles are separated there from the mixture and the fines leave the housing with the gas flow via a bore of the rotor and an outlet tube.
In the known classifiers, the separation zone is short and of ineffective spatial configuration. Therefore, the particles coming accidentally outside its limits are excluded from the process of separation, and not all the particulate material lends itself to whirling by the discs of the known design. Appreciable variations in particle size at the fraction border are unavoidable in this case, and particles of a size less than 5-10 .mu.m are inseparable.
Also known is a classifier (cf. Patent of FRG No. 3303078, Cl. B07B 7/083, 1983) which incorporates a housing contained wherein is a rotor with coaxially attached discs linked to each other by blades which serve to whirl an air/powder mixture fed between the discs over an inlet tube of the housing. The outlet for the separated material is via a bore of the rotor and an outlet tube.
Here, the separation zone is even smaller than one described above, being confined only to the outside surface of the separating means, so that uncomfortably high rotor speeds of about 20000 rpm are required in order to separate particles with a size of 4-8 .mu.m.